Feathers missing on her neck could be caused by molting. Chickens often start molting their neck feathers first. But, if you're seeing your Leghorn pecking her, that might be the cause of the feather loss (but it could also be the effect--she might have seen a bare area, and then started pecking).

I would isolate the Leghorn from the other birds, as long as no other birds are picking on your Araucana. Keep her isolated for two weeks, and then introduce her back in. She will probably be the lowest in the pecking order when she goes back with the flock, so it will be a while before she can start picking again.

If other birds are picking at her, isolate the Araucana. Keep her isolated until the feathers grow back in, which usually takes about six weeks.